Python Script - A Tool not found

I have a python tool (termed myScript.py here). The related syntax of this script is below:
myGDB = "C:/.../wayside_access_LRS.gdb/"
arcpy.gp.toolbox = myGDB + "Toolbox";
arcpy.gp.AddLRSSegmentID(RailLRSDef_Fix, myGDB + output_Table)
The error message is: AttributeError: Object: Tool or environment <AddLRSSegmentID> not found.
In the wayside_access_LRS.gdb/Toolbox, the another script tool: AddLRSSegmentID is there.
Is anything wrong in my python code? Thanks if you can point out.